The present invention relates to a charged particle beam apparatus, and particularly to an electrification prevention technique for a wafer in an apparatus for inspecting and measuring the size and shape of a pattern formed on a sample.
Inspection and measurement of the conditions of a circuit pattern formed on a semiconductor wafer during a process of manufacturing a semiconductor device is a technique that plays an important role in improving a yield and establishing reliability. The inspection and measurement is realized by an apparatus using lights or an apparatus using charged particle beams. Particularly, along with miniaturization of a recent semiconductor device, inspection and review using charged particle beams represented by electron beams in a critical dimension scanning electron microscope (CD-SEM) becomes important. However, since a surface of the semiconductor wafer during the manufacturing process is configured by an insulating film made of silicon oxide, silicon nitride, an organic material, a high-dielectric material or the like, when the charged particle beam is irradiated onto the surface of the wafer, the surface of the wafer is electrically charged, and accordingly the orbits of signal particles generated from the surface are changed, resulting in deterioration of a scanned image to be obtained.
Various causes of the image deterioration are conceivable. One of them is an image distortion caused by electrification on the surface of a sample such as a wafer, as shown in FIG. 1. Specifically, an area whose image is already obtained is positively-charged by emission of secondary electrons, and when a scanned image for another area adjacent the area is to be obtained, the orbits of primary electron beams are affected by positive charges of the adjacent area, and the scanned image to be generated is distorted by the detected secondary electrons, resulting in local magnification variation.
As shown in FIGS. 2A and 2B, due to the electrification of the surface of the sample, a focus electric field of the primary electron beam by an objective lens is affected, so that the primary electron beam is changed from a focus state (FIG. 2A) to a defocus state (FIG. 2B), thus deteriorating the resolution of a scanned image to be obtained.
Further, as shown in FIG. 3, the secondary electrons generated from the sample by the positive electrification of the surface of the wafer are attracted by the positive electrification of the surface of the wafer to return to the wafer. Accordingly, efficiency of detecting the secondary electrons varies, resulting in darkness of an entire scanned image to be obtained or local reduction in brightness.
In order to avoid the affect of the electrification on the sample, for example, Japanese Patent Application Laid-Open No. 2005-345272 proposes electrification reduction by a boosting method. In the boosting method, a voltage (boosting voltage) applied to a booster (boosting) electrode incorporated into an objective lens of an SEM is set in accordance with a pattern shape and a material of a sample to be observed, so as to change the electric field intensity of the surface of the sample.